My GDrive (S/N W506100208) is out of warranty and it appears the HDD failed, so after speaking with a GTech Support person I purchased a new Seagate barracuda 2 TB SATA HDD and installed it in the drive enclosure. Everything worked fine; Mac recognized the drive, I formatted it for both Mac & PC using the Mac Disk Utility, no issues. Ejected the drive, shut down the computer, next morning the Mac is not seeing the drive and the drive bay's light is flickering rapidly. I connected it to a PC and it did not recognize it either. What could be the issue?

Thanks for posting. I take it it is the same on both firewire and usb? Is the drive seen in any management tools such as disk management or device manager on windows, or disk utility or system report on mac?

It could be that the controller for the enclosure has not gone out as well. There could also be the possibility of a conflict with the drive you installed as they have only been tested with HGST drives. The former seems more likely though.